The miniaturization and weight reduction of electronic devices are being promoted. Meanwhile, soldering temperature in assembling electronic devices tends to rise. Consequently, aluminum electrolytic capacitors used for electronic devices require miniaturization and greater capacitance with thermal stability against leakage of an electrolyte solution and a short circuit.
High temperatures may cause electrolyte leakage from an aluminum electrolytic capacitor. Aluminum foil for a negative electrode and a lead wire connected to the foil electrochemically react with the electrolyte solution at high temperatures, thereby increasing the amount of hydroxide in the electrolyte solution near the lead wire. Deterioration of a sealing member due to the alkalization is supposed to be the cause of electrolyte leakage.
Meanwhile, miniaturization and greater capacitance require the surface area of a negative electrode to be expanded by etching, and thus aluminum foil containing 0.1% to 0.5% of copper is generally used for a negative electrode. However, if copper contained in the negative electrode is dissolved into the electrolyte solution and is re-precipitated, a short circuit occurs.
International Patent Publication No. 95/15572 discloses an electrolyte solution produced by dissolving amidine salt made of an alkyl substituent amidine group and carboxylic acid, into a solvent containing γ-butyrolactone, as an electrolyte solution with less leakage. It also describes that the electrolyte solution improves the low-temperature characteristics of an aluminum electrolytic capacitor and excels in thermal stability. However, this type of aluminum electrolytic capacitor, if aluminum foil containing copper is used for the negative electrode, causes the above-described short circuit particularly in a high-temperature, high-humidity environment.
Meanwhile, Japanese Patent Unexamined Publication No. H02-77109 discloses a technique for preventing re-precipitation of copper by forming complex salt with copper dissolved from aluminum foil, using an electrolyte solution including a solvent containing ethylene glycol and others with ethylenediaminetetraacetic acid added thereinto. However, ethylenediaminetetraacetic acid forms complex salt also with aluminum ions dissolved into the electrolyte solution, and thus may insufficiently prevent a short circuit due to dissolution and re-precipitation of copper.